This invention relates to the cleanup of flue gases produced from power plants, and, more particularly, to removal of both particulate and gaseous pollutants from the flue gas.
In a fossil-fuel power plant, coal or oil is burned to boil water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the combustion produces gaseous pollutants such as sulfur and nitrogen oxides, and a solid particulate termed fly ash. Environmental protection laws mandate that the amounts of the gaseous and solid pollutants be maintained at acceptably low levels. The present invention deals with reducing and maintaining both the smog-producing nitrogen oxides, known generally as NOx, and the particulate fly ash within acceptable levels.
A number of technologies are currently employed to reduce the amount of NOx and particulates in the flue gas stream before the flue gas is exhausted to the atmosphere. Particulates are removed from the combustion gas by mechanical devices such as cyclones or barrier filters, or by electrostatic precipitators. Electrostatic precipitators may be placed into the hot gas stream either upstream or downstream of the combustion air preheater. The former is known as a hot-side electrostatic precipitator and the latter is termed a cold-side electrostatic precipitator.
NOx may be removed by reacting the NOx with ammonia. This reaction is accomplished either at high temperature, in the gas phase with no catalyst present, or at lower temperature, at the surface of a catalyst. The catalyst is typically coated onto a fixed substrate bed through which the flue gas flows.
While these techniques are effective to some extent, it is always desirable to provide a further reduction in the pollutants within the limits of economic feasibility. The present invention provides such a further reduction.